Many wells for the production of crude oil and gas are located in offshore waters. These wells can be found in inland waters as well. However, in the continuing search for petroleum reserves, the offshore type well is used most frequently in petroleum reserves found further and further from shore.
Often, where a highly productive offshore field is discovered, a number of wells will be drilled into the area, each being furnished with a well head which protrudes just above the ocean floor. Often in a highly productive area, these well heads are judiciously spaced and interconnected through suitable manifolding or preferably through pipelines. The latter are generally directed to a common assembly point such as a collecting platform set at a central location, and provided with means to receive and initially treat the petroleum product.
The nature of the ordinary well head is such that it embodies sufficient control means that the well can be adjusted to safely produce at a desired rate. Depending on the type of well being utilized, the well head itself can assume different proportions and embody varying forms of equipment adapted to the particular subsea conditions.
In either instance, the well head is generally set on a pad which in turn is piled to the ocean floor. Physically the well head extends upwardly from the pad for a sufficient distance to provide the necessary producing requirements.
A further feature of the well head is that it must accommodate workover tools and instruments which are periodically inserted into the well for various reasons. The well head therefore constitutes an essential part of the producing function and is a highly important piece of equipment.
Since offshore wells are located in many different water depths, they are subject to damaging effects peculiar to the particular area. These danger sources include not only the peril of underwater objects which might be floated into contact with the well head, but also objects which are dropped or lowered from the surface.
A primary source of damage to a well head resides in the fishing nets and trawling boards equipment used by commercial fishermen who fish the water in the vicinity of the wells. These nets can be highly damaging to a well head when they become entangled. Further, they are capable of severely bending, or at least inflicting minor damage to well head parts. The latter, although built for strength, are nonetheless susceptible to being distorted when subjected to a severe pulling force.
Another source of possible danger to offshore wells stems from the use of anchors and anchor chains. It can be appreciated that in the instance of a heavy chain, the latter could become snagged or at least partially wrapped around a part of the well head. Thus, when the chain is retrieved, it could distort or even break parts from the well head.
The undesirable consequence of such an underwater emergency would be an uncontrolled flow of the crude oil or gas. There have been instances for example when well heads have been damaged by anchor chains to the point where the well has become unusable.
It has therefore become a desirable, if not mandatory, practice to provide some form of protective coverage for exposed sea bottom well heads. Preferably, such protective units are designed to deflect or deter damaging contact between a well head and heavy moving objects or lines. Thus, a protector's primary function is to permit the well head to operate in a safe manner and yet be readily accessible for workover purposes and/or for diver inspection.
One problem that arises during the positioning of well head protectors is the difficulty encountered in accurately lowering the protector from a floating vessel to bring it properly about the well head. This lowering function can be hazardous, and is always expensive. For example, unless the condition of the ocean is relatively calm, and the vessel subjected to a minimal degree of movement, lowering of the well head protector over one side or the aft end, even with the aid of guide cables, can be uncertain and hazardous.
Installing a well head protector is not a simple operation and normally requires the use of special derrick equipped vessels which are capable of transporting the protector as a unit to the well site and thereafter lowering it over one side. The daily rental cost of such vessels can be exorbitant and in the instance of bad weather the mere rental of the vessel while waiting for favorable conditions, can constitute a substantial expense.
To overcome the foregoing problems, there is presently provided a well head protector which is sufficiently heavy to perform its designed function, and yet can be readily installed at a subsea location. The protector is thus initially fabricated into discrete sub-assemblies which can be easily transported to an offshore drilling vessel located above the well head.
The individual protector parts are assembled at the vessel into a singular unitary structure as the latter is progressively lowered beneath the vessel. This avoids the necessity for putting the protector together on the vessel deck. In one embodiment, the vessel is of the semi-submersible type which is adapted for deep water drilling. Thus, its deck can be raised and lowered in the water by controllable buoyancy tanks.
With the protector fully assembled, it can be conveniently lowered by the vessel's drilling equipment to the well head, by way of guide wires which extend to the latter from the vessel. The protector, during the lowering operation, is supported from the drilling vessel's derrick and/or drill string and is laterally restrained by guide lines.
It is therefore an object of the invention to provide a well head protector, and a method for installing the latter about a well head located at the ocean floor. A further object is to provide a well head protector that can be readily assembled into a unitary structure after the discrete parts thereof have been transported to an offshore well head site. A still further object is to provide the means, and a method for assembling and lowering a well head protector from a drilling vessel located at the site of a well head to be covered.